The present invention relates generally to electronic sunglasses, and, in particular, to transmittance-varying sunglasses which employ the electro-optical effect of a liquid crystal material and a solar cell which serves both as a power source for driving the liquid crystal and as a sensor for detecting the quantity of ambient light.
The provision of transmittance-varying sunglasses using a liquid crystal material has been proposed in various publications. The general concept has been illustrated in, for example, in FIG. 1 of Japanese Patent First Publication No. Showa 48-98846, FIG. 3 of Japanese Patent First Publication No. Showa 51-124936, FIG. 1 of Japanese Utility Model First Publication No. Showa 62-127524, FIGS. 5 and 7 of Japanese Patent First Publication No. Showa 62-40425, FIG. 1 of Japanese Utility Model First Publication No. Showa 55-95106, FIG. 1 of Japanese Utility Model First Publication No. Showa 59-178618, and FIG. 1 of Japanese Utility Model First Publication No. Showa 62-109137. As has been understood from these examples, such electronic sunglasses are composed of a liquid crystal panel serving as a transmittance-varying section, a power source, a driving circuit, a control circuit, and an optical detecting section. However, these prior art constructions have proved less than completely satisfactory for the following reasons.
Considering transmittance-varying sunglasses employing the electro-optical effect of liquid crystal and using a solar cell as a power source, the prior art sunglasses include a circuit for detecting an electromotive force of the solar cell and switching the transmittance mode according to the amount of ambient light. In the case where the operating illumination of the liquid crystal panel is the same as the stopping illumination, the received illumination is often varied according to the changing orientation of a user and obstacles which screen the light from the sun rays, resulting in frequent switching of the liquid crystal panel from an operation mode to a stop mode or vice versa and varying transmittance of a lens. This frequent variation is a great annoyance to a user.
It is very important to set an illumination at which transmittance is varied. Hence, unless the illumination is properly set, the change of transmittance is not performed in a preferable illumination, resulting in annoyance to a user. To set the illumination properly, the prior art devices provide a means for varying the resistance value in a circuit, which makes it possible to set an illumination. Yet, it is very troublesome to arrange a variable resistor according to the environment in which the sunglasses are used. No constructions have been proposed for setting a user-friendly illumination at which transmittance is changed. Further, in the case of setting a proper illumination, in some conditions, it may be desirous to make transmittance lower below the set illumination to change the transmittance from a high mode to a low mode or to make transmittance higher over the set illumination to change the transmittance from a low mode to a high mode. This means that it is insufficient to provide the change of transmittance at the two stages of a high mode to a low mode or vice versa of the received illumination. This is a relevant shortcoming to be overcome.
Except for the system for switching the liquid crystal panel on or off at the set illumination as mentioned above, there have been other systems for arranging transmittance by continuously changing a voltage applied to the liquid crystal panel according to the quantity of outside light and for arranging transmittance by changing a ratio of an on time to an off time during one period of a pulse to be applied to the liquid crystal according to the quantity of outside light. These systems, however, are not effective in some conditions. For example, an uneven density appears on the liquid crystal panel by changing dependency on a view point based on the change of a voltage applied to the liquid crystal. When, for example, a user drives through the woods by car, that is, the quantity of ambient light is abruptly and frequently changed, the transmittance is frequently changed. This frequent change is a great annoyance to the user.
Accordingly, it is desired to provide electronic sunglasses which are capable of preventing frequent change of transmittance, allowing proper setting of the set illumination according to the situation with one touch, and changing transmittance according to a user's will.